Piezoelectric materials exhibit strain when subject to an applied electrical field. For example, piezoelectric materials exhibit both elongation and contraction when subject to varying electrical fields. As a result, piezoelectric materials have been used in actuator applications where their linear relationship of strain to the applied field is exploited to create both elongation and contraction, thereby creating a bi-directional actuator.
Recent advances in the fields of piezoelectric and ferroelectric research have lead to the discovery of new materials exhibiting even larger but electrostrictive or contractive strain. In particular, electrostrictive crystals such as Lead Magnesium Niobate-Lead Titanate [PMN-PT], Lead Zinc Niobate-Lead Titanate [PZN-PT], and electron-irradiated copolymer poly (vinylidene fluoride-trifluoroethyline) [P(VDF-TrFE)], exhibit large energy densities and recoverable strains of 1% to 4%. In general, the higher strain capacity and energy density of electrostrictive materials (as compared to piezoelectric materials) make them attractive replacements for piezoelectrics in actuators. For example, the strain coefficient for the piezoelectric polyvinylidene fluoridine is less than 10% of the strain coefficient for the electrostrictive irradiated P(VDF-TrFE).
However, electrostrictive materials only exhibit unidirectional strain even when polarity of the applied electrical field is reversed. Due to this feature, only unimorph or one-directional electrostrictive actuators have been created. Unimorph electrostrictive actuators typically include a passive restoring layer. This passive layer reduces the active portion of the actuator, and thus decreases the total energy density of the actuator. If the entire actuator could be active, an electrostrictive bimorph or bi-directional actuator could theoretically exceed the performance of a similar piezoelectric bimorph actuator by a factor of at least 5. However, a fully active bimorph electrostrictive actuator would have to compensate for the material having only unidirectional strain.
Therefore, there is an unmet need for bimorph or bi-directional actuators using electrostrictive materials.